You trashed it? I don't think that's possible since the trash turns into eject when you drag a volume and you can not eject the boot volume.
You either dragged it off the sidebar in a Finder window. In that case just drag it from the Desktop into the sidebar or use Finder->Preferences->Sidebar to turn it on again.
Or you turned off the display of volumes on the Desktop. You can turn that on again in Finder->Preferences->General